Răspuns :
S= 5 - 2 * 5 +3 * 5 - 4 * 5 .....+99*5 - 100 * 5
S= 5*(1-2+3-4+5-6+7-8.....+99-100)
S= 5*(-50)
S= -250
S= 5*(1-2+3-4+5-6+7-8.....+99-100)
S= 5*(-50)
S= -250
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x+y-2(x+y)+3(x+y)-4(x+y)+.....+99(x+y)-100(x+y)si acum avem 5-2×5+3×5-4×5+...+99×5- 100×5=5(1-2+3-4+5-6+....+99-100)=5(-1+3-4+5-6+....+99-100)=5(2-4+5-6+....99-100)=5(-2+5-6+.....99-100)